The present disclosure relates to poured concrete slabs, such as poured concrete slabs for buildings.
A foundation of a building connects the building to the ground, transfers loads from the building to the ground, and provides structural support for levels of the building above the foundation. A multistory building has a greater mass, and thus frequently requires multiple load-bearing elements to provide greater surface area to transfer load and provide structural support. One example of a load-bearing element is a wall, which is at least partially embedded into the ground. Another example of a load-bearing element is a column supported by a footing. The footing is embedded in the ground beneath the building and transfers weight from the load-bearing column to the ground beneath the building.
In a building that includes a basement, the foundation is formed below the surface of the surrounding ground. Thus, to form the basement, earth at the site of the building is typically excavated and graded, and the load-bearing elements, such as walls and columns, are inserted into the excavated space. The walls define the shape of the basement, and soil may be backfilled around the walls to return the earth surrounding the basement to grade.
Once the walls and columns of the foundation are in place, a concrete slab is typically poured within the walls and around the columns. The walls of the basement provide the formwork for the poured concrete, which is typically poured onto a base of gravel or crushed stone, to promote drainage, or onto the subsoil. When the poured concrete has solidified, the resulting concrete slab is not part of the foundation, but does provide a solid concrete floor surface to the basement.
As shown in FIG. 1, a typical basement 10, as described above, is shown from a top view. As shown, the walls 14 of the basement 10 may include reentrant corners 18, which accommodate structural features of the building on levels above the basement and provide greater surface area to distribute the load of the building to the ground. Each of the reentrant corners 18 is formed at an intersection between adjacent walls 14, extends inwardly into the basement 10, and subtends an angle of greater than 180°. In other words, each reentrant corner 18 forms the center of a circular arc, which is indicated in FIG. 1 by dashed line 20. Each of the circular arcs 20 extends from one of the adjacent walls 14 of the corner 18 to the other. The circular arc 20 of each of the reentrant corners 18 extends greater than 180°, thus each of the reentrant corners 18 subtends an angle of greater than 180°. In the example shown in FIG. 1, the walls 14 form three reentrant corners 18, each of which subtends an angle of approximately 252°.
The basement 10 also includes columns 22, which can be used to support main floor beams of a post and beam system. The columns 22 are spaced apart from the walls 14 and from one another to further facilitate load distribution by the foundation elements. Wet concrete is poured into the enclosed space formed by the walls 14 and around the columns 22. The poured concrete conforms to the shape between the walls 14 and columns 22, and when the poured concrete solidifies, it forms a concrete slab 26, which provides a solid concrete floor surface to the basement.
One difficulty that is encountered with poured concrete slabs is that the concrete is prone to cracking at the reentrant corners, such as reentrant corners 18 shown in FIG. 1, as well as at support columns, such as columns 22 shown in FIG. 1. As the poured concrete hardens or cures, water evaporates from the concrete, and the concrete shrinks while also adhering to the surrounding walls. As the concrete shrinks and adheres to adjacent walls 14 at a reentrant corner 18 or adjacent sides of a column 22, the concrete is pulled in two orthogonal directions. Thus, reentrant corners 18 and columns 22 concentrate tensile stresses in the concrete. Additionally, the concentration of tensile stresses at reentrant corners 18 and columns 22 reoccurs when the concrete shrinks and expands due to temperature and moisture changes over the lifetime of the concrete slab. In addition to tensile stresses within the concrete as the concrete cures, curling tensile stresses can also occur on the top of the slab during the first few hours after slab placement. These tensile stresses also pull the slab apart at corners.
When the tensile stresses applied to the concrete exceed the tensile strength of the cured concrete, the cured concrete cracks. A crack usually extends diagonally from the apex of each 252° reentrant corner 18 at an angle of about 135°. In other words, the crack usually extends at an angle that is generally equidistant from the adjacent walls 14. As further shrinkage and curling occur, the crack widens and lengthens. Because linear shrinkage and curling continue at a decreasing rate for more than two years, cracks can become quite wide and long. For example, for a concrete slab that is 6 inches thick, the shrinkage that occurs after one year is only 60-80% of the ultimate shrinkage, which means cracks that appear early in the lifetime of a slab can grow significantly over time.
One common strategy to reduce the risk of cracking has been to make small diameter saw cuts in the concrete that are aligned with the structural walls at reentrant corners. For some structures, such as dock pits, doweled construction joints with plate dowels can be used that allow differential movement parallel to the doweled joint. In another approach, diagonal reinforcing bars are embedded at reentrant corners. The reinforcing bars will not prevent the crack from occurring, but will help keep the crack tighter, or narrower, and shorter.
These and other typical strategies to reduce cracking require significant time and/or materials to implement. Some existing strategies may be suitable for industrial applications, such as dock pits for industrial warehouses and loading docks, where cost and construction time limits are not prohibitive. However, in high-volume, low cost scenarios, such as home building, these prior strategies are too expensive and take too much labor and time to implement. Consequently, there is a significant need for a quick, inexpensive, and effective solution to reduce cracking of poured concrete slabs.